Beginning the simple.c import.
Added some issue # references.
Fixed two instance domain bugs.
Removed an unnecessary tempoary variable.
Refactored the initialize_pmda code for slight performance improvements and (more importantly) tracking of the assigned instance domain IDs; also fixed an important bug in on_fetch_callback that was rejecting valid metric requests.
Cluster, item and instance domain types are all unsigned (see __pmID_int); not sure how I missed this originally.